You might be in a destination where no 3G network is available i.e. Australia, Japan, Puerto Rico, Taiwan or USA. Check these settings to make a call.
1. Log in to My Singtel app and tap on Roaming.

2. Tap on the gear icon and enable Data Roaming (Network).


3. If you are using a travel eSIM from other operators, go to your device settings and Turn on this line and Mobile data.
If you also have a Singtel SIM, make sure that you've turned off Data roaming in your device settings.
Note that pay-per-use charges will apply for any calls made.
